Image taken by J.R. Hott of Panhandle Helicopters of Panama City, Fl. on Sunday 5 Feb.

Panhandle Helicopter tours posted this pic on their Facebook page Sunday, and I sure wish I’d been on the ride! What is happening here is fairly easy to explain and it has to do with the wind, the condos, and the moisture in the air.

Parcels of air cool at a known rate when lifted. Once they become saturated they cool at a slower rate because heat is released as water vapour condenses. Image cosy. Wash. State Uni.

Cool air offshore was very nearly at the saturation point, with a temperature near 20ºC and a dew point of about 19.5ºC. The air at this temperature can only hold a certain amount of water vapour, and how much it can hold depends heavily on the temperature. If you add more water into the air, a cloud will form, but you can also get a cloud to form by cooling the air. Drop the temperature, and it can no long hold as much water vapour, so some of it will condense out and a cloud will form.

In this case, the air was cooled by lifting it about 50 meters over the top of the condos. A parcel of unsaturated air will cool when lifted at a rate of 1ºC per 100 meters. In this case, it probably cooled about 0.5 degrees C, but that was all it took! On the back side of the condos, the air slowly sinks back down and warms at the same rate. As it warms the air can hold more water vapour and the cloud evaporates and disappears!

I chatted with J.R. Hott, the owner of Panhandle Helicopters, and he tells me that he sees this effect a few times a year. This one was one of the best, because many times it fogs in before the can get his chopper in the air to grab a snap! If you find yourself down that way on a day that is a bit misty, head on over and take a ride. You just might see it for yourself!

Knowing what is happening makes it even more neat to look at IMHO. Physics in action!

Update: The Bernoulli effect also is at play here and might even be the dominant player. The acceleration of the wind over the buildings causes the pressure to drop and that causes the temperature to drop. See the comments below by Bob Reed for more details.

Comments

19 Comments

Dan! It’s been a while, I hope all is well with you. Thanks to your blog, this image caught our attention at The Weather Channel. We’re airing it tonight with an explanation from Dr. Greg Forbes. (7p/10p est)

Hi Dan,
Your explanation is very good, but I disagree that the cooling effect is due so much to the air rising as it is to an aerodynamic effect.

In an adiabatic flow, as the velocity of the flow increases the static pressure of that flow decreases; with it, so does the temperature. This effect is quantified via Bernoulli’s equation.

So as the flow expanded from offshore to move around, and over, the buildings it’s velocity necessarily increased in the same fashion as moving over an aircraft’s wing. Concomitantly, the pressure (and temperature) decreased, and, VIOLA!, you have the fascinating condition that the helicopter pilots captured in these vivid photos.

It’s very much like when you look at images of a high-performance aircraft maneuvering, and you see a blanket of condensation covering the wing.

My comments is in no way meant to be pedantic, friend, but I’m an aerospace engineer that recognizes that this is kind of a freebie flow visualization sans wind tunnel; instead provided solely by mother nature

I think you have a valid point Bob. Was actually wondering if I would hear from some engineers about that aspect of it. I thought about the Bernoulli effect but was unsure how much wind speed would be needed to make a significant enough drop in pressure/temp. Guess we could run Bernoulli’s eq. for say 7 m/sec and see how much of a press/temp. drop there is.

Have seen the vapour trail effect many times on wet days in the air. Perhaps a combination of both?

We have great exhibition on Bernoulli at the Sci-Quest museum here in Huntsville and I suspect other science museums do as well. I once did a project on it way back in uni. days at Okla. Uni. We measured the wind speed and pressure drop around a tall building on the OU campus!

Hi Dan. I enjoyed the pictures very much. The phenomenon is quite fascinating. I would question your explanation about the “water holding capacity” of air, however. Air has such a low density that there is plenty of “room” for as much gaseous water as you please. We’ve known for hundreds of years that gases do not affect other gases in air. It is not like dissolving salt in water where there is a limited amount or space and one can eventually reach a “point of saturation.” Instead, there are two things happening at the same time. There is evaporation which is temperature dependent and there is condensation which is P H2O dependent. At a constant temperature, there is a constant rate of evaporation, if the P H2O should rise, increasing condensation rate to exceed the rate of evaporation, clouds form. Or, as discussed by Bob Reed, above, pressure decreases, slowing condensation, but temp must also decrease to an extent which lessens the rate of evaporation to below that of condensation and, again, you have clouds.
The idea of a holding capacity of air for water is a very common misconception, found in many text books even, but it is important, I think, to get the physics right. For more information on this misconception, I would direct your attention to the “bad meteorology” website: http://www.ems.psu.edu/~fraser/BadMeteorology.html

I really do wish that we’d end the explanation that air (consisting mostly nitrogen and oxygen) has some magic capacity for “holding” water. The use of words such as “hold” or “capacity” are misleading when describing water vapor content. Air does not really “hold” water vapor in the sense of making “room” for it. Here’s a better explanation: http://www.ems.psu.edu/~fraser/Bad/BadClouds.html.

Technically you are correct. I have struggled with this for quite sometime, but I’ve come down on the side of continuing to use it because more water can evaporate into warmer air. I have on past posts linked to the website in your comment above. This issue might make a good post. If there are some mets who agree with my take on it out there and want to make there case, I will post both!

maybe the phenomenon is much more simple. It could be compared with flow over mountains. In this case, the tower buildings function as orographic obstacles and force the air to rise up.

Aside from it, a very flat inversion should be present to form these stable-stratified clouds. Unfortunately, I didn’t find any representative sounding ascents in the vicinity of the city. However, the other pictures clearly reveal an unstable layer above, with deep convection well above the fog clouds.

Hi, Dan
Being a retired meteorologist I’ve seen a lot of orographic fog in the Greenland area years ago, and I completely agree with Felix. It’s obvious from the picture that there is a thin mist layer over the relatively cold sea surface, and this mist condenses into fog due to adiabatic cooling when lifted over the buildings…
Greetings from Denmark
Leif Rasmussen

About Dan

Dan Satterfield has worked as an on air meteorologist for 32 years in Oklahoma, Florida and Alabama. Forecasting weather is Dan's job, but all of Earth Science is his passion. This journal is where Dan writes about things he has too little time for on air. Dan blogs about peer-reviewed Earth science for Junior High level audiences and up.MORE ABOUT DAN >>

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